This invention relates to apparatus and methods for cooling power transformers.
Electric utilities use large power transformers to distribute power (voltage and current) along and within their distribution territory. These xe2x80x9cpowerxe2x80x9d transformers handle large amounts of power (e.g. 10 million volt-amperesxe2x80x9410 MVA) and are normally made to have very low winding resistance (Rw). However, at elevated load currents (I), the power dissipation (I2Rw losses) in the transformer winding translate into the generation of large amounts of heat which in turn causes the temperature of the power transformer to rise. Accordingly, the temperature of these power transformers increases as a function of the load (power drawn from the transformer) and ambient temperature.
The power drawn through a transformer may increase significantly due to a fault on a distribution line or some other overload condition. In addition, a large increase in the power drawn through a power transformer may occur due to certain operating procedures as illustrated with reference to FIGS. 1 and 2.
FIG. 1 shows a block diagram of a substation 10 used to distribute power from a primary source 12 to various loads connected to the substation. Input power from source 12 is coupled via circuit breaker CB1 to a transformer T1 and is distributed via a closed switch SW1 to a station bus section line 14 from which power is then distributed via circuit breakers CB4 and CB5 to feeder lines F1A and F1B to which loads L1 and L2 are, respectively, connected. Likewise, input power coupled via circuit breaker CB2 to a transformer T2 is distributed via a closed switch SW2 to a station bus section 16 from which power is then distributed via circuit breakers CB6 and CB7 to feeder lines F2A and F2B to which loads L3 and L4 are, respectively, connected.
From time to time the load from one transformer (e.g., T2) is switched to another transformer (e.g., T1) in accordance with some standard operating procedure, e.g., whenever it is necessary to service power lines or equipment inside and/or outside the substation. By way of example, this is illustrated with reference to FIG. 2 when switch SW2 is opened and bus tie breaker CB3 is closed. Then, all the currents for loads L1 through L4 are drawn from T1. Because of the increased loading on the transformer (e.g., T1), the temperature of the transformer will increase with time and may rise above the ambient temperature by a significant amount. Insofar as T1 is concerned this load condition would represent a xe2x80x9chighxe2x80x9d load condition.
Conventional cooling systems rely on sensing the temperature of the power transformer and/or other points representative of the actual transformer temperature. If and when the temperature being sensed rises above a predetermined level, a cooling system is activated; where, for example, the cooling system may include banks of fans blowing air over the transformer or pumps causing cooling oil to be circulated about the transformer windings. However, it should be noted that the power transformers are physically massive devices which have large thermal time constants (e.g., one-half hour). Thus, by the time the free air maximum rating temperature of the transformer is sensed and the cooling system is activated, the temperature of the winding will continue to rise and may exceed the xe2x80x9cratingxe2x80x9d temperature of the transformer. The temperature of the transformer and its windings may thus continue to rise above critical values giving rise to xe2x80x9cservice lifexe2x80x9d problems, as discussed below.
It is important to maintain the temperature of a power transformer at, or below, certain specified temperature ratings because the service life of the transformer is reduced when these specified temperature ratings are exceeded. By way of example, at elevated temperatures the winding insulation begins to breakdown Also, the circulating oil may break down and/or volatile gases may be produced creating potentially hazardous conditions. To ensure that the temperature rating of the transformer is not exceeded a variety of cooling systems (e.g., forced air or circulating oil) may be used, as already noted, to ensure that the winding temperature of the transformer stays below its specified ratings.
As noted above, known methods for controlling the temperature of a power transformer includes sensing the temperature of the transformer and/or making direct temperature measurements of selected points associated with the transformer and then turning on fans for blowing air onto the transformers or causing cooling oil to be circulated. This is not satisfactory because of the potentially large thermal overshoots.
It should also be appreciated that operating the cooling system on a continuous basis is expensive and increases the wear and tear on the cooling equipment. Therefore, it is undesirable to operate the cooling system continuously if such operation is not needed. On the other hand, as just noted, the delay in energizing the cooling system causes the temperature to overshoot which in turn reduces the life of the transformer.
The problems present in the prior art are mitigated using apparatus and methods embodying the invention. In accordance with the invention the load current drawn from a power transformer is sensed (by sensing the current in the primary or secondary of the transformer) to determine if and when the current exceeds a predetermined threshold. The length of time the load current exceeds the threshold is also sensed. By monitoring the excess current flow and the length of time for which it flows, it is possible to anticipate a rise in the temperature of the transformer and in its winding and to initiate cooling before the transformer and its windings have reached a critical temperature. Thus, systems embodying the invention include means for sensing the current drawn from and/or by a transformer, determining when the current exceeds a predetermined value and timing means for sensing the length of time for which the excess current flows. The timing means are needed, in part, to differentiate between a transitory overload condition and a static, continuous, high load condition.
Applicant""s invention thus bypasses the long thermal time constant and enables an appropriate cooling response to be initiated at an early point of a heat cycle which can prevent the transformer temperature from rising significantly above its rated value, thereby extending the useful life of the transformer.
Thus, in systems embodying the invention the turn-on of the cooling system is made a function of the electrical power dissipation which causes heat dissipation which in turn causes a rise in the temperature of the transformer. This is in sharp contrast to the prior art schemes where the temperature of various surfaces or items is sensed to determine when the cooling system is to be turned on.